1. Field of the Invention
The present invention relates to a process for producing terephthalic acid. More particularly, the invention relates to a process for producing high purity terephthalic acid suitable for use as starting materials to obtain a high quality polyester by direct polymerization with glycols.
2. Description of the Prior Art
A so-called "direct polymerization process" in which terephthalic acid is directly reacted with glycols (e.g., as described in U.S. Pat. Nos. 3,451,910 and 3,406,153) is an improvement over the complicated process for producing polyethylene terephthalate by subjecting terephthalic acid to an esterification to obtain dimethyl terephthalate and then conducting an ester exchange reaction with glycols and a polycondensation reaction (e.g., as described in U.S. Pat. Nos. 2,647,885 and 2,578,660). In the direct polymerization process the terephthalic acid used must have a very high purity.
Therefore, the terephthalic acid obtained by oxidizing an alkylated aromatic hydrocarbon with molecular oxygen in the presence of a catalyst comprising a heavy metal compound and a bromine compound (or bromine) using a lower aliphatic carboxylic acid as a solvent, which was developed by Scientific Design Co. (and is known as the "SD process") and is industrially widely employed as described in the specification of Japanese Patent Publication No. 2666/1956 (corresponding to the original specification of a continuation-in-part application, U.S. Pat. No. 2,833,816) contains reaction intermediate products such as 4-carboxybenzaldehyde, p-toluic acid, etc. and reaction by-products having an undetermined indefinite structure in a large amount and cannot be used in a direct polymerization without purification. Hence, it is necessary to purify terephthalic acid obtained by the SD process to achieve a high purity using various purification techniques which are complicated and uneconomical. It would be most advantageous if high purity terephthalic acid could be obtained directly by a liquid-phase oxidation of an aromatic hydrocarbon such as p-xylene, in a single step.
In a liquid-phase oxidation reaction for producing terephthalic acid, various methods of improving the activity of the catalyst by limiting the ratio of the heavy metal component and the bromine component which are used for the catalyst, the concentration thereof, etc. to a preferred range have been heretofore proposed as described, for example, in Japanese Patent Publications 15531/1967, 329/1970, etc., but these methods can be characterized as employing as reaction conditions relatively mild conditions of atmospheric pressure and low temperature (in general, around the boiling point of reaction solution, and where acetic acid is used as the solvent, about 118.degree. C.). Where an oxidation reaction is conducted under such mild reaction conditions there is the disadvantage that the reaction rate is markedly reduced and, as a result, to obtain a yield the same as that obtained in the process using conventional high temperature and pressure conditions the intent is to limit the ratio (amount) of catalyst used and the concentration thereof to a specific range thereby to increase the reaction rate. However, terephthalic acid of satisfactory purity and color for use in the direct polymerization is not obtained.